There are various types of gas laser beam generators, such as high speed axial, biaxial transverse, and triaxial transverse types. All of these, however, are equipped with both rear mirrors and output mirrors, and generally, the action of changing the mirrors in the resonator, especially the rear mirror, is very troublesome. The reason is that, in many cases, the laser head is incorporated in the machine, and in such a case there is inadequate working space, so that considerable time is required to complete the work. In addition, an already known power monitor is generally mounted on a rear mirror holder at the laser head. In a mirror having this type of construction, there is the problem that the normal changes in atmospheric pressure added to the thickness of the mirror body tend to distort the mirror. At the present time, the mirror body is generally made thicker as a measure to prevent distortion of the rear mirror.